Trench isolation is commonly used in semiconductor integrated circuits. Trench isolation provides improved isolation performance with regard to leakage and breakdown voltage. Trench isolation also takes less space than other methods such as junction isolation or LOCOS (Local Oxidation of Silicon).
The desire to save space causes trenches to be etched with high aspect ratios. A high aspect ratio for a trench means that the trench has a large vertical depth relative to the lateral width of the trench. Trenches that have a high aspect ratio are often filled with polysilicon that is deposited by low pressure chemical vapor deposition (LPCVD). LPCVD polysilicon is highly conformal and is able to coat the walls and bottom of a trench far from the surface of a wafer.
Polysilicon has the added advantage that it has the same thermal expansion coefficient as the surrounding monocrystalline silicon in which the semiconductor device is built. However, because polysilicon is not an insulator, the polysilicon must be separated from the adjacent monocrystalline silicon areas on either side of the trench with an insulator. The insulator that is most often used is a thermally grown silicon dioxide layer.
During subsequent processing steps the silicon dioxide layer can cause problems. One problem is that the silicon dioxide layer can be attacked during wafer etching and precleaning. This may cause portions of the silicon dioxide layer to be eroded. Erosion of the silicon dioxide layer can leave a gap between a monocrystalline silicon portion and an adjacent polysilicon portion. The gap can trap undesirable materials, thereby compromising the isolation properties of the trench and causing the semiconductor device to fail.
Another problem is that the silicon dioxide layer provides a diffusion conduit for oxidizing species (e.g., oxygen or steam) during subsequent thermal processing. This causes oxidation along the lateral seam on both sides of the polysilicon of the trench structure as well as on the adjacent walls of monocrystalline silicon. Because silicon dioxide occupies approximately twice as much space as the space of silicon that it consumes, the volume of material in the oxidized region of the trench expands substantially. This expansion creates stresses that cause defects (e.g., cracks) in the monocrystalline silicon areas. These defects can lead to leakage within the semiconductor device that the trench is designed to isolate. It is possible that the defects can even lead to the failure of the semiconductor device.
Therefore, there is a need in the art for a system and method for providing improved trench isolation for semiconductor devices in an integrated circuit. There is a need in the art for a system and method for manufacturing an isolation trench that does not expose the seams of polysilicon portions and adjacent monocrystalline silicon portions to subsequent etching and oxidation.